[I am responding to a post in comp.lang.c++, but the subject is
C, so the response is directed to comp.lang.c.]
Keith Thompson <
Keith.S.Thompson+u@gmail.com> writes:
wij <wyniijj5@gmail.com> writes:
[...]
typedef int (*ptr)(); // ptr is pointer to int function
int H(ptr x, ptr y);
int D(ptr x)
{
int Halt_Status = H(x, x);
if (Halt_Status)
HERE: goto HERE;
return Halt_Status;
}
int main()
{
H(D,D);
return 0;
}
The code above does not compile:
Yes, it does (as you acknowledge in a later post).
This:
typedef int (*ptr)();
defines "ptr" as an alias for a type that can be described in English
as "pointer to function returning int". The empty parentheses
indicate that the function takes an unspecified but fixed number
and type(s) of arguments; this is an old-style declaration. [...]
The function H is declared but not defined. [...]
I'll note that the code (declares and) defines the function D,
but never calls it. The address of D is passed to H, but without
a definition of H we can't guess what it does with that address.
All good up to this point.
It's possible to rewrite the code to (a) avoid the use of old-style
function declarations [...]
Not without radically altering the code. Because D is passed
as an argument to H, and because of how H and D are declared,
the type of (the address of) D has to match the type of D's
first parameter. It isn't possible to do this in C without
resorting to a partial function type somewhere (because the
type of D is infinitely recursive).
There are other ways to work around this problem, for example
involving having a pointer-to-function member in a struct,
but that would entail changing either the declarations or
the call of H() with D as an argument, which seems to be
stretching the bounds of "rewrite the code".
The code as presented is a valid C *translation unit*, but it
is not a valid *program*,
It might be better to say it's a valid partial program, but not
a complete program.
and it has no behavior.
It doesn't have behavior in the sense that it doesn't do anything.
It does have behavior in the sense that the C standard prescribes
a meaning for the declarations and definitions given. I believe
the C standard uses the term "behavior" in both of those senses.
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